Description of the Prior Art
During the manufacture of semiconductor devices, such as VLSI devices, semiconductor wafers that are being processed are exposed to liquids in a liquid bath or spray, for example. Typical liquids include deionized (DI) water, hydrogen peroxide, hydrochloric acid, hydrofluoric acid and sulphuric acid. Contamination of a liquid occurs when spurious particles enter the processing environment and become mixed in the liquid being used. These contaminants can contact and be deposited on the wafers, which would result in defective wafers thereby significantly lowering production yield.
In prior art systems, light scattering is used to detect particles by means of a very bright light beam that is directed through the liquid being sampled. If particles are present in the liquid, the light is scattered to a photodetecting means. Similar types of methods have been used to count airborne particles, such as described in the textbook by William C. Hinds, entitled "Aerosol Technology", published by John Wiley & Sons, Inc., New York, 1987.
It is known that sensitivity in liquids is much less than air, because the amount of light scattered scales as the ratio of the index of refraction of the particle to the index of refraction of the liquid medium. Although it is possible to detect particles in the 0.1 to 0.2 micron diameter size range in air, the typical detection limit in liquids is in the 0.3 to 0.5 micron size range. Therefore, it has been difficult to detect small contaminant particles that appear in liquids used for processing wafers.
An additional problem that exists in prior art particle detection systems for liquids is the presence of bubbles in the liquid. A bubble appears as a particle to the detector and thus it is generally difficult to distinguish bubbles from particle contaminants by simple light scattering technology. The conventional method to eliminate bubbles is to pressurize the liquid. This approach requires testing of the liquid sample, rather than monitoring the liquid used in the process continuously. Also, a pressurization apparatus is complex, expensive and may be dangerous to use. An improved system for distinguishing bubbles from particles in a liquid is disclosed in the aforementioned patent application.
In some particle detectors used for monitoring liquids, a nebulizer is used to aspirate the liquid and form an aerosol of small droplets. Solid particles are obtained that can be detected. In known systems that employ nebulizers, the user needs to add liquid manually to the nebulizer, by means of an eye dropper, for example. This method does not afford the monitoring of the liquid on a real-time continuous basis.